Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geologic formation, i.e., a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. Once a wellbore has been drilled, the well must be completed; a completion involves the design, selection, and installation of equipment and materials in or around the wellbore for conveying, pumping, or controlling the production or injection of fluids. After the well has been completed, production of oil and gas can begin.
In deep wells, reliability of the sand face completion is very important, due to the prohibitively high cost of intervention. Further, many such wells are completed open hole. Therefore, gravel packing of open-hole horizontal wells is increasingly becoming a standard practice in the deep-water, sub-sea completion environment. The gravel packing process involves pumping into the well a carrier fluid that contains the necessary amount of gravel. Consequently, gravel is deposited adjacent an open hole, for instance at one end of the wellbore, where it serves to prevent sand from flowing into the wellbore.
Proper selection of the carrier fluid is essential. Ideally, the carrier fluid shall not cause any permeability reduction of the formation. When viscous fluids are used, carrier fluid must also have sufficient viscosity to suspend and carry the gravel during placement. Carrier fluids are either considered “water-based” or “oil-based” depending on the constituency of their external continuous phase; the term “oil based” fluids may thus be used to designate fluids having a continuous phase based on synthetic or non-synthetic oil and eventually, an aqueous phase dispersed in the oil phase, said aqueous phase being more properly designed as a brine.
Aqueous-base fluids can be tailored to be compatible with most formations simply by including salts such as potassium chloride or ammonium chloride. Consequently, to date, the convention in gravel-packing such horizontal wells has been water packing or shunt-packing with water-based viscous fluids. A carrier fluid is thus typically a brine comprising a gelling agent such as hydroxyethylcellulose (HEC), xanthan or a viscoelastic surfactant; breakers to minimize the pressure required to move the fluid back to the wellbore.
However, aqueous gravel pack fluids are poorly compatible with oil-based drilling fluids. To ensure compatibility and improve wellbore cleanup, a common practice is to use a water-based reservoir drilling fluid for the section of the well to be gravel-packed. However, the use of a water-based drilling fluid is not always suitable or the best recommended practice. Indeed, oil-base drilling fluids provide shale inhibition, lubrication, gauge hole, and higher rates of penetration lubricity and deeper bit penetration and therefore, may often be preferred over water-base fluids.
Solids-free brine-in-oil emulsions for well completion are known from SPE Paper 17161. These emulsions consist of an aqueous brine phase dispersed as fine droplets in a base oil, where the droplets are stabilized through a surfactant/emulsifier that forms films around the water droplets produced when water is added with shears to the oil phase containing the emulsifier. The emulsion viscosity is linked to the number and volume of droplets present and also the emulsifier concentration. In this paper, the used emulsifier is a polymerized nonionic surfactant with lipophilic and hydrophilic groups attached to a straight backbone. Its molecular weight is between 6,000 and 10,000.
An oil-based emulsion gravel-pack carrier fluid was also reported in SPE Paper 64978 that further discusses the possibility of adding a chelating agent solution to the internal phase that is for instance released by breaking the emulsion. This paper further depicts the effect of mixing energy on emulsion viscosity and the sand carrying properties of this oil-based emulsion fluid. However this paper is silent as to the choice of the emulsifier and the authors of the present invention have found that proper selection of said emulsifier was indeed critical for providing an effective oil-based emulsion gravel-pack carrier fluid. It is well known that commercial emulsifiers such as fatty ac id esters are often blends of the mono, di, tri etc. esters, and also may be prepared under uncontrolled or different reaction conditions with differing temperatures, reactants, catalysts, etc. Thus, there is a need for a way to predict or determine whether or not a particular emulsifier will be effective in an oil-based emulsion gravel-pack carrier fluid.